ix.j PALEONTOLOGY AND EVOLUTION. 199 



of these orders is represented in the Miocene epoch : 

 the Eocene formation, as I have already said, contains 

 Cheiroptera, Insectivora, Rodentia, Ungulata, Carnivora, 

 and Cetacea. But the Cheiroptera are extreme modifica- 

 tions of the Insectivora, just as the Cetacea are extreme 

 modifications of the Carnivorous type ; and therefore it 

 is to my mind incredible that monodelphous Insecti- 

 vora and Carnivora should not have been abundantly 

 developed, along with Ungulata, in the Mesozoic epoch. 

 But if this be the case, how much further back must 

 we go to find the common stock of the monodelphous 

 Mammalia ? As to the Didelphia, if we may trust the 

 evidence which seems to be afforded by their very scanty 

 remains, a Hypsiprymnoid form existed at the epoch of 

 the Trias, contemporaneously with a Carnivorous form. At 

 the epoch of the Trias, therefore, the Marsupialia must 

 have already existed long enough to have become differ- 

 entiated into carnivorous and herbivorous forms. But 

 the Monotremata are lower forms than the Didelphia, 

 which last are intercalary between the Ornithodelphia 

 and the Monodelphia, To what point of the Palaeozoic 

 epoch, then, must we, upon any rational estimate, rele- 

 gate the origin of the Monotremata ? 



The investigation of the occurrence of the classes 

 and of the orders of the Sauropsida in time points in 

 exactly the same direction. If, as there is great reason 

 to believe, true Birds existed in the Triassic epoch, the 

 ornithoscelidous forms by which Eeptiles passed into 

 Birds must have preceded them. In fact there is, even 

 at present, considerable ground for suspecting the exist- 

 ence of Dinosauria in the Permian formations ; but, in 

 that case, lizards must be of still earlier date. And if 

 the very small differences which are observable between 

 the Crocodilia of the older Mesozoic formations and 

 those of the present day furnish any sort of approxi- 



